1. Field of the Invention
The present invention is directed to an image forming apparatus including a recording head that jets liquid droplets.
2. Description of the Related Art
Among image forming apparatuses such as printers, facsimile machines, copiers, plotters and multifunction peripherals having the aforementioned functions for performing image formation, there are liquid jet recording image forming apparatuses including a recording head for jetting, for example, ink droplets. An ink jet recording apparatus is known as an example of such liquid jet recording image forming apparatuses. The liquid jet recording image forming apparatuses jet ink droplets from the recording head onto a sheet being transferred to form an image on the sheet. It is to be noted that the term “sheet” in the present application is not limited only to paper, and refers to a medium onto which ink droplets or another type of liquid is allowed to adhere. Examples of such a medium include an OHP film. The term “sheet” may be referred to also as “recording target”, “recording medium”, and “recording sheet”. Furthermore, in this application, the terms “recording”, “printing” and “imaging” are used synonymously with the term “image forming”. There are different types of liquid jet recording image forming apparatuses, such as, a serial-type image forming apparatus which forms an image by causing a recording head to jet liquid droplets while moving in the main scanning direction; and a line-type image forming apparatus which forms an image by causing a line-type recording head in a stationary position to jet liquid droplets.
It is also to be noted that the term “image forming apparatus” in the present application refers to an apparatus for forming an image by jetting liquid onto a medium made of, for example, paper, textile threads, fibers, fabric, leather, metal, plastic, glass, wood or ceramic. In addition, the term “image forming” includes forming not only an image having meaning (e.g. characters, figures and symbols) but also an image having no particular meaning (e.g. patterns) on a medium. In this sense, simply depositing liquid droplets on a medium is also regarded as “image forming”. The term “ink” is not only directed to substances called ink, but is used as a generic term for all liquid substances allowing image formation, such as recording liquids and fixing liquids.
Such liquid jet recording image forming apparatuses, particularly ones that form an image by causing a carriage having a recording head for jetting liquid droplets to travel in a reciprocating motion (i.e. moving alternately backward and forward), have the following problem. That is, in the case of printing bidirectionally, positional misalignment tends to occur if the printed image is a ruled line. Also, in superposing different colors, a color registration error is likely to occur.
In the case of ink jet recording apparatuses, these problems are handled generally in such a manner that the user selects and inputs optimal values with reference to an output test chart for adjusting misalignment of landing positions of liquid droplets so that the jetting timing is adjusted based on the input results. However, the test chart is subject to individual interpretation, and data input errors may occur due to inexperienced users, thus possibly posing greater problems in the adjustment.
In order to address the problems associated with the test chart, conventionally, a test pattern is formed on a conveying belt or a media conveying member and then read by a sensor (see, for example, Patent Documents 1, 2 and 3).    [Patent Document 1] Japanese Examined Patent Application Publication No. H4-39041    [Patent Document 2] Japanese Laid-open Patent Application Publication No. 2005-342899    [Patent Document 3] Japanese Patent No. 3838251
Patent Document 4 discloses a technique for forming on recording paper a test pattern, which is then read by a sensor.    [Patent Document 4] Japanese Laid-open Patent Application Publication No. 2004-314361
Patent Document 5 discloses a technique in which a positional misalignment correction pattern is formed on a conveying belt and then read by a sensor for detecting the presence or absence of the positional misalignment correction pattern. A filter process is subsequently performed on an output of the sensor using a filter for cutting off frequency components higher than a frequency of the positional misalignment correction pattern. Patent Document 5 discusses that positional misalignment can be corrected by removing high-frequency component noise in this manner.    [Patent Document 5] Japanese Patent No. 3640629
However, in the case of forming a test pattern on a conveying belt or a medium and reading it by a sensor as described above, it is difficult to accurately read the test pattern if there is a small difference between, for example, the color of the conveying belt and the color of an ink used. In order to achieve accurate color detection, a structure is needed such that colors are detected using, for example, light sources having different wavelengths corresponding to respective colors, however, in practice, conventional techniques cannot accurately read the test pattern formed on the conveying belt.
For example, assume that the conveying belt is an electrostatic adsorption belt including an insulating layer on its surface and a medium resistance layer on its rear surface, and carbon is mixed in the medium resistance layer to provide conductivity. In this case, the color of the conveying belt is black, and therefore, pattern detection by measuring only color reflectance has little success since the conveying belt cannot be distinguished from black ink.
In order to resolve this problem, the following technique for accurately detecting the position and positional misalignment of the pattern may be considerable. First, a pattern is formed on a water-repellent pattern formation member so that the pattern is made up of isolated ink droplets. The ink droplets have the characteristic of being separately formed in a hemispherical shape. Using this characteristic, a single-wavelength light beam is projected onto the pattern on the pattern formation member. The specularly reflected light of the projected light beam attenuates over the pattern with the ink droplets, whereby the position and positional misalignment of the pattern can be accurately detected.
However, if a conveying belt, for example, is used as the water-repellent pattern formation member, the surface of the conveying belt changes over time. It is also subject to accidental scratches and dirt build-up caused by paper-dust and paper-jam removing operations. By simply eliminating high-frequency component noise as described in Patent Document 5, low-frequency noise cannot be removed that are caused due to such accidental scratches and dirt as well as the time degradation of the belt, thus interrupting accurate pattern detection.
In view of the above-described issues, the present invention aims at maintaining at a stable level pattern detection accuracy and accuracy of correcting the liquid droplet landing positions.